Although early theorizing about the self conceptualized it as a unitary structure (e.g., AIIport, 1955; Rogers, 1951), more recent treatments have proposed that it is multifaceted and context-dependent (Baumeister, 1998; Linville & Carlston, 1994). But how does the self operate in this manner? Are there qualitative differences among multiple selves and their attributes that affect mental and physical health? And most important, what processes explain the functions of multiple selves? This project uses a self-complexity framework to examine these issues. People are greater in self-complexity when they report having more self-aspects (e.g., roles, relationships) that are comprised of a greater degree of unique attributes. It has been proposed that those lower in self-complexity are more likely to reveal spillover effects. That is, self-relevant feedback has greater impact for those lower in self-complexity because the feedback implicates a relatively larger proportion of the overall self that is comprised of relatively more interrelated attributes. For instance, research has shown that those lower in self-complexity are more vulnerable to depression and stress-related illnesses when facing life stressors (Linville, 1987) and have difficulty in suppressing unwanted self-relevant thoughts (Renaud & McConnell, 2002). However, some findings have been inconsistent and even contradictory (e.g., Rafaeli-Mor & Steinberg, 2002; Woolfolk et al., 1995), suggesting that our understanding of self-complexity is far from complete. Critical assumptions about self-complexity (e.g., Linville, 1985) remain untested, and no comprehensive model speaks to the processes by which multiple selves form, function, and influence mental health and social behavior. These experiments (3 preliminary, 10 proposed) explore three lines of work to improve our understanding of how self-complexity relates to well-being. First, three studies assess the mechanisms underlying spillover effects. A second series of studies examine important qualities of self-aspects and their attributes to develop a more complete account of how self-complexity relates to depression, stress-related illnesses, self-esteem, and mood swings. Preliminary data indicate that perceptions of personal control moderate the relations between self-complexity and well-being, and additional work explores how the importance of multiple selves, self-certainty, and personality factors shape the relation between self-concept representation and well-being. Finally, the third line of research proposes a brand new model of self-representation derived from Associated Systems Theory (AST; Carlston, 1994), providing a very different and novel conceptualization of multiple selves, measure of self-complexity, and process account for spillover effects. Preliminary data suggest this AST-based approach has utility, and additional work explores whether AST provides a more comprehensive approach to self-complexity than traditional treatments. This work will increase our understanding of the interplay between multiple selves and one's overall sense of self. Moreover, we will develop better measures of multiple selves and self-complexity, test and extend existing theory on self-concept representation, develop clearer insights into the relations between multiple selves and well-being that can address inconsistent findings in the literature, and suggest new avenues for research that illustrate the importance of understanding how the representation of multiple selves mediates affective responses to life events.